Pressure gradient responsive microphone



Jan. 11, 1955 A. H. KETTLER ET AL PRESSURE GRADIENT RESPONSIVE MICROPHONE 2 Sheets-Sheet 1 Filed Nov. 13 1950 w W i M ZALFRE D\H. KETTLEH i.

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. H AWLEY AHoRNEY Mum-:3 BY

Jan. 11, 1955 KETTLER ETAL 2,699,473

PRESSURE GRADIENT RESPONSIVE MICROPHONE Filed Nov. 13, 1950 2 Sheets-Sheet 2 //YV/V7'0/?5 ALFRED I'I. KETTLER $2 Mums E.HAWLEY Y ATTORNEY United States Patent Ofifice 2,699,473 Patented Jan. 11, 1955 PRESSURE GRADIENT RESPONSIVE MICROPHONE Alfred H. Kettler and Mones E. Hawley, Collingswood, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application November 13, 1950, Serial No. 195,204 3 Claims. (Cl. 179-115.5)

The present invention relates to sound translating apparatus, and more particularly to a microphone of the pressure gradient responsive type.

Under varying circumstances in transmission of sound, it is very desirable to provide a high degree of background noise discrimination to assure etficient transmission of speech or other sound sources in close proximity to the sound translating apparatus. Pressure gradient responsive microphones have been found particularly useful for such purposes.

The pressure gradient responsive microphone depends for its operation upon the difference in sound pressure between two points, for example, differences in sound pressure on opposite sides of a vibratile member or diaphragm.

A pressure gradient responsive microphone is one the output of which is substantially proportional to a derivative of the sound pressure with respect to distance. Microphones of this type are classified according to the order of the pressure derivative. Thus, a first order microphone has an output proportional to the first derivative. A second order microphone has an output proportional to the second derivative. An nth order microphone has an output proportional to the nth derivative. A first order, pressure gradient responsive microphone may comprise either two elements responsive to the pressure of a sound wave, or a single element responsive to the pressure gradient of a sound wave. A second order, pressure gradient responsive microphone would require either two first order microphones, or four pressure microphones. A third order microphone would require either two second order microphones, or four first order microphones, or eight pressure microphones. An nth order microphone would thus require 2" pressure microphones. For a more complete description of these microphones, reference may be made to the patent to Olson, 2,301,744.

Microphones of the type above referred to, particularly those of the higher order type, are difficult to construct because all of the elements involved must have substantially identical sensitivities and frequency response characteristics. It is apparent, therefore, that it would be desirable to have a sound translating instrument of the higher order type with as few elements as possible.

It is a primary object of the present invention to provide a microphone having a single pressure-sensitive element the frequency response characteristic of which is tlgat of a second order, pressure gradient responsive microp one.

It is another object of the present invention to provide an improved structure for a pressure gradient responsive microphone having a minimum number of parts and which is highly eflicient in operation.

A further object of the present invention is to provide a noise-cancelling microphone of the second order, pressure gradient responsive type which will have a large ratio of signal to random noise with a minimum number of I arts.

A still further object of the present invention is to provide a noise-cancelling microphone of the second order, pressure gradient responsive type which has a large ratio of signal to random noise and which requires but a single pressure-sensitive element.

Another object of the present invention is to provide a noise-cancelling microphone of the second order, pressure gradient responsive type which requires but a single pressure-sensitive element and which is mechanically symmetrical.

lt is also an object of the present invention to provide a microphone having a single pressure-sensitive element the frequency response characteristic of which is that of a third order, pressure gradient responsive microphone.

Still another object of the present invention is to pro vide a noise-cancelling microphone of the third order, pressure gradient responsive type which will have a large ratio of signal to random noise with a minimum number of parts.

A further object of the present invention is to provide a noise-cancelling microphone of the third order, pressure gradient responsive type which has a large ratio of signal to random noise and which requires but a single pressuresensitive element.

Another object of the present invention is to provide a noise-cancelling microphone of the third order, pressure gradient responsive type which requires but a single pressure-sensitive element and which is mechanically symmetrical.

The pressure gradient responsive microphone of the present invention comprises a casing having a single pres sure-sensitive element mounted therein with separate air chambers or spaces disposed on opposite sides of the pressure-sensitive element. Depending upon the particular order of microphone operation desired, sound transmitting means in the form of tubes or passages in the microphone casing are provided which communicate with the air chambers. The impedances of each of the sound transmitting means bear ratios one to another determined by the order of gradient operation desired.

The novel features characteristic of the present invention, as Well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawings in which,

Figure l is a side elevation of a second order, pressure gradient responsive microphone, in accordance with the present invention,

Figure 2 is a front elevation of the microphone shown in Figure 1,

Figure 3 is an enlarged, sectional view of the motor elements of the second order microphone, taken on the line 33 of Figure 1, portions of the microphone casing being broken away,

Figure 4 is a wiring diagram of a simplified electrical circuit equivalent to the acoustical network of the vibrating system of the second order microphone shown in Figure 1,

Figure 5 is a side view, partly broken away, of a third order, pressure gradient responsive microphone, in accordance with the present invention,

Figure 6 is a top plan view of the third order microphone shown in Figure 5, and

Figure 7 is a wiring diagram of a simplified electrical circuit equivalent to the acoustical network of the vibrating gystem of the third order microphone shown in Figure Before referring to the accompanying drawings in greater detail, it is desired to point out the background theory leading to the develo ment of the present invention. In accordance with this theory, it can be shown that fewer elements will suffice for pressure gradient re soonsive microphones than those heretofore provided. Suppose, for example. that one wishes to build a second order microphone. t four pressure microphones are used, let their respective pressures be designated as P1, P2, P3, P4. Their method of combination for second order operation would be as follows:

By utilixing the second pressure P2 twice, one can obtain, for the first pressure difierence (Pl-P2) and for the second pressure difference (P2P3). These may be combined and expressed as follows to obtain second order operation:

(P1P2)(Pz-P3) 2 Expression 1 may be simplified and written as follows: P12P2+P3 Similarly for a third order microphone, instead of using eight pressure microphones only five are necessary since one can utilize pressures P2, P3 and P4 twice. Thus, for the first pressure difference one obtains (P1-Pz), for the second pressure difference one has (Pa-P3), for the third pressure difference one has (Pa-P4) and for the fourth pressure difference (P4P5); and expressed as follows toobtain third order operation:

It will be noted that the third pressure P3 cancels out. Therefore, it is not necessary to utilize the microphone providing pressure P3 at all. Second and third order, pressure gradient responsive microphones are used herein merely'as illustrative'examples'. Accordingly, it'will be recognized that microphones of higher orders maybe investigated in similar manner.

By deduction, a general explanation for the numberof pressure-sensitive elementsand'the method of their combination may be obtained so that annth order microphone may be thought of as consisting of 2" order microphones each of which consists of a pair of 2"- order microphonesand so-on. Thus, for any order of operatiomnit will be observed that-each pressureis used twice except the first and last whichare used only once. Now with respect to the first pressure the otherpressures may be additive or subtractive depending on whether they are used in the samesense min the opposite sensewith the first pressure. Indeed, in some cases, the pressure may be additive in one place and subtractive in another. This'is the case when the pressure'is used as the last'pressure'of a microphone of even numbered order gradient operation and the first pressure of the next microphone of the same ordergradient operation. For example, reference is made to Expression 4 above wherein P3 is thelast'pressure of the other second order, pressure gradient responsive microphone. So far as the resultant pressure is concerned, pressures which are added in one place and sub tracted in another may be omitted entirely providing the proper spatial distribution of the other pressuresis not changed. These results maybe expressed in a formula for the resultant pressure pn, as follows:

Pizsound pressure at the-1st position in space; 8:1, 2, 4, 5, 6, 8, or all positive integers except those assumed by the Expression 1+2 (1+2m),

where r=0, 1, 2, and m=0,.l, 2"

ts=the element of the sequence of increasing nonnegative integers (0, l, 2 which corresponds to S ing non-negative integers (0, 1, 2, in one-to-one correspondence with the set of values of .S;

nzthe order of gradient operation; and V P K+1 =sound pressure at the (K+1)th position in space.

On the basis of this theory, a second order, pressure gradient responsive microphone may be constructed which uses but a single pressure-sensitive element. One such embodiment which was constructed and successfully operated as a second order microphone is illustrated in Figures 1 through 3 of the drawing. In this particular embodiment, the second. order microphone 1 comprises a housing or casing 3 of a design suitable for holding in a users hand. The upper end of the housing3' is provided with a compartment 5 within which a sound translating unit 7 may be mounted by any suitable mounting means.

The sound translating unit 7 comprises a magnetic field structure 9 including a cup-shaped field yoke 11 of permeable magnetic material, a core 13 of permanent magnet material, and an annular outer pole piece 15 of soft iron or other suitable materialof high permeability. The core 13 is concentrically mounted within the cup-shaped yoke 11, being attached to the base or closed end 19 thereof by any suitable mounting means. The annular outer pole piece 15 is provided with a central aperture 21' and is mounted at the open end of the field yoke 11 being attached adjacent the peripheral edge 23 thereof These may be combined by suitable attaching means. The inner pole piece 24 is mounted on thecore 13 for disposition concentrically within the outer pole piece central opening 21 and in spaced relation to the outer pole piece 15 so as to provide an annular air gap 25 therebetween. A flexible, disclike diaphragm or. pressure-sensitive member 27 is supported at its periphery between a pair of annular spacing rings 29 of non-magnetic material, such as brass, or the like, whichare. attached to the yoke periphery 23. A cylindrical voice coil form 31 carrying a voice coil 33 has an end thereof attached to the central portion of the diaphragm 27. The voice coil forrn 31 is arranged with the voice coil 33 located within the air gap 25 for freedom of movement simultaneously with vibratory movement of the diaphragm 27 so-that the field structure 9 serves to convert responseof the pressure-sensitive member 27, due to sound energy impinging thereon, into cor:- responding electrical signals.

According to Expression 3 above, the diaphragm 27 should be twice as sensitive to a unit quantityof acoustic energy to one side thereof as it is to a unit quantity of acoustic energy directed to the other side thereof. Acoustic energy may be directed to the proper side of the diaphragm 27 through short open tubes or other passages leading'from orifices in the housing to acoustic air chambars 34', 35 disposed, respectively, on opposite sides of the diaphragm adjacent to front and rear surfaces 36, 37 thereof. In the particular embodiment shown, openings 38 are provided at diametrically opposite points in the fieldyoke 11v for communication with the rear surface 37 of the diaphragm throughthe space 39 within the field yoke, the air gap 25, and the acoustic chamber 35 adjacent the diaphragm rear surface 37. Two openings 43 are. provided in the housing 3 at oppositely disposed points communicating with the compartment 5. The openings 43 are arranged to communicate with the open ings 35 provided in-the field yoke 11. Additional openings 45 are provided in a front wall 47 of the housing and serve to transmit acoustic energy to the front surlgace3356of= the diaphragm 2.7 through the acoustic cham- In order toprotect the elements of the sound translating unit 7from dust particles, a filter or screen 49 comprising a layer of felt or other suitable material is disposed between thehousing 3 and the field yoke 11 so as to covcr the openings 38 in the field yoke. Inasmuch as the openings45 in the front wall 47 of the housing 3 are those into which a user of the microphone would normally direct his speech, a membraneous material 51 is disposed over the openings to keep out moisture. The. membraneous material 51 will also serve to keep dust particles from entering the microphone. The material 51 should be sufficiently light in weight and loose so. as not to interfere with sound waves directed to the front surface 36 of the diaphragm 2,7. The cavity or space 39 within the. field yoke 11 is filled with wax 53, or other suitable material to prevent resonance effects.

The acoustical circuit for the second order microphone may beillustrated bya simplified electrical circuit, as shownin Figure 4, wherein:

R1=the lumped acoustical resistance of one of the openingsleadingtorthe rear surfaceof the diaphragm;

Rz=the lumped acoustical resistance of the opening leading to the front surface of the diaphragm;

R3=the lumped acoustical resistance of the other opening. leading to the rear surface of the diaphragm;

Rn=the lumped acoustical resistance of the diaphragm;

Li=the lumped acoustical inertance of one of the openings leading to the rear surface of the diaphragm;

Lz=the lumped acoustical inertance of the openings leading to the front surface of the diaphragm;

L3=the lumped acoustical inertance of the other opening leading to the rear surface of the diaphragm;

Ln=the lumped acoustical inertance of the diaphragm;

C1=the lumped acoustical compliance of the air chamber adjacent the rear surface of the diaphragm;

C2=the lumped acoustical compliance of theair chamber adjacent the front surface of the diaphragm;

Cn=the lumped acoustical compliance of they diaphragm;

V1=the sound pressure at one of. the openingsleading to the rear surface of the diaphragm;

Vz=the sound pressure at the openings leading to the front surface of the diaphragm;

Vs=the sound pressure at the other opening leading to the rear surface of the diaphragm; and

in=the acoustical volume current through the diaphragm.

If the identical sound pressures are incident upon all three orifices at the same time, the diaphragm 27 will not move. This is substantially the case for random noise. In the equivalent circuit this condition will be realized when V1=V2=Vs, and in=0. 'An analysis of this network shows that this condition is satisfied when the fol lowing ratios of acoustical impedances exist;

Therefore, the openings 43, 45 leading to opposite sides of the pressure-sensitive element are dimensioned so that the sum of the products of the acoustical impedances of the openings leading to field yoke cavity and the acoustical impedance of the field yoke cavity is substantially equal to four times the product of the acoustical impedance of the other opening and the acoustical impedance of the air chamber 35 communicating with the diaphragm front surface 36. Thus,

This may be accomplished, for example, by making the openings 38, 45 substantially uniformly spaced apart, or by making the openings 38 leading to the rear surface of the pressure-sensitive member 27 of substantially equal acoustical impedance and the acoustical impedance of the other opening 46 substantially equal to one-half that of each of the two openings 38, or by making the acoustical impedances of all the openings 38, 46 substantially equal, or by other arrangements which will satisfy the Equation 7.

It will, of course, be recognized that any one of the openings may comprise a plurality of smaller openings provided the total acoustical impedance of all the smaller openings is equal to that of the single opening. For example, as seen in Figure 2 of the drawing, the eifectively single opening leading to the front surface of the diaphragm may comprise a plurality of small openings 45 the total impedance of which is such as to be identical with the impedance for a single opening which satisfies the above Equation 7. Thus, the minimum number of openings required for a second order microphone is three, with the openings disposed in series relation. The extreme openings 38 of the series are connected with the acoustic chamber 35 communicating with the rear surface 37 of the diaphragm, while the other opening (the plurality of openings 45) is connected with the acoustic chamber 34 communicating with the diaphragm front surface 36.

Although it is not necessary to keep the distance between the successive points at which the pressure is observed constant in order to obtain operation of a given order, if these distances are not the same the calculation of the proper impedance relationships becomes more difficult.

However, the distances between the apertures should also be as large as possible in order to obtain high sensitivity; the limit on this distance is the effective phase differences between the points at which the pressure is observed when the distances between these points becomes an appreciable part of the wavelength. Therefore, insofar as both sensitivity and fidelity are concerned, it is generally most efficient to maintain the distances between the points at which the pressure is observed a constant.

A third order, pressure gradient responsive microphone may also be constructed using a single pressure-sensitive element. According to Expression 4, five pressure-sensitive elements must be considered in the case of a third order microphone. The elements of Expression 4 may be denoted as the first, second, third, fourth and fifth elements respectively. Although it is necessary to take into account all five elements, Expression 5 shows that only four elements are actually necessary. The output of the third element is not used but is taken into account in the spacing of the other four elements. In other words, a third order microphone may be provided by admitting acoustic energy to a single pressure-sensitive element through four apertures (denoted herein as first, second, third and fourth elements corresponding, respectively, to the pressure elements disposed at the first, second, fourth and fifth positions in space according to Expression 5) and directing sound energy to one side of the diaphragm through the first and third apertures, to the other side of the diaphragm through the second and fourth apertures, and making the diaphragm movement twice as sensitive to a unit quantity of energy entering either the second or third apertures as to a unit quantity of energy entering either the first or fourth apertures. In accordance therewith, a third order microphone, illustrated in Figures 5 and 6, is provided which comprises a housing or casing 54 having a single pressure-sensitive member or diaphragm 55 mounted therein in such a manner as to be responsive to sound pressure impinging on opposite surfaces thereof. The housing 54 comprises an enclosure for the diaphragm 55 formed by two cylindrical shaped members 57 disposed on opposite sides thereof and held together by any suitable means such, for example, as the annular, threaded collar 59. Annular spacing members or washers 61 are disposed between the diaphragm 55 and the housing members 57 at the periphery of the diaphragm 55 thereby to provide acoustic chambers 65, 67 on opposite sides of the diaphragm.

Tubes or conduits 69 are provided for picking up sound I energy at the required pressure pick up points and for transmitting that energy to opposite sides of the diaphragm 55 in the proper ratio of impedances. The tubes 69 are arranged to correspond to the first, second, third and fourth elements mentioned above. One end 71 of the tubes is attached to the housing 54 in any suitable manner which will connect the tubes with the acoustical chambers 65, 67. Their opposite ends 73 are open and arranged to terminate one after the other along a line at spaced intervals corresponding to the spatial distribution required for the first, second, fourth and fifth positions in accordance with the requirements of Expression 5. The tubes 69 are made of substantially equal lengths and uniform internal cross-section so that they will provide substantially equal acoustical impedances.

In order that the tubes occupying the second and third positions in the series will provide twice the sensitivity to the diaphragm that the first and fourth tubes provide, two tubes are employed at those positions. Effectively, the two tubes at the second and third positions function as a single tube the acoustical impedance of which is equal to one-half the acoustical impedance of that of either tube at the first or fourth positions. In other words, the single first and fourth tubes in the series which occupy extreme positions, provide twice the acoustical impedance that the tubes occupying the second and third or mean positions do. The tubes 69 are connected to the acoustical chambers 65, 67 in such a manner that those tubes occupying the first and third positions in the series are connected with the acoustical chamber on one side of the diaphragm 57 and the tubes occupying the second and fourth positions are connected with the acoustical chamber 67 on the opposite side of the diaphragm. Thus, the tubes are connected alternately in the series with different acoustical chambers.

An electro-mechanical converter 74 of suitable design is mounted on the exterior of the housing 54 by any convenient attaching means. The converter is provided with an armature 75. The armature 75 is connected to the diaphragm 55 by a drive rod 77. The drive rod 77 extends through an opening 79 in the housing 54 which is sealed acoustically by a small rubber or other suitable gasket 81. Upon movement of the diaphragm 55 in response to a difference in sound pressure on opposite sides thereof, the armature 75 will vibrate directly with the diaphragm and function to produce an electrical signal in a manner well known in the art.

The acoustical circuit for such a third order microphone may be illustrated by a simplified electrical circuit, as shown in Figure 7, wherein:

R1==the lumped acoustical resistance of the first opening or pressure-sensitive point;

Rz=the lumped acoustical resistance of the second opening or pressure-sensitive point;

R3==the lumped acoustical resistance of the third opening or pressure-sensitive point;

R4=the lumped acoustical resistance of the fourth opening or pressure-sensitive point;

R =the lumped acoustical resistance of the diaphragm;

L1==the lumped acoustical inertance of the first opening or pressure-sensitive point;

L2=the lumped acoustical inertance of the second opening or pressure-sensitive point;

Ls=the lumped acoustical inertance of the third opening or pressure-sensitive point;

L4=the lumped acoustical inertance of the fourth opening or pressure-sensitive point;

L ==the lumped acoustical inertance of the diaphragm;

C1=the lumped acoustical compliance of the cavity on one, side of the diaphragm;

C2=the lumped acoustical compliance of the cavity on, the other side of the diaphragm;

C the lumped acoustical compliance of the diaphragm;

V1=the sound pressure at the, first opening or pressure sensitive point leading to one surface of the diaphragm;

Vz=the sound pressure at the second opening or pressure-sensitive. point leading to the other surface of the diaphragm;-

V3:--the sound pressure at the third opening or pressure-sensitive point leading to the first mentioned surface of the diaphragm;

V4=the sound pressure at the fourth opening or pressure-sensitive point leading to the second mentioned surface of the diaphragm;

i =the acoustical volume current through the diaphragm.

The conditions upon the sensitivity of the diaphragm may then be stated as follows: (1) if the sound pressure at all the apertures is identical at the same time, the voltage output will be zero; and (2) if sound pressure p (where p=any arbitrary sound pressure) is incident upon the second aperture and the pressure at all the other apertures is zero, the displacement of the diaphragm will be twice that obtaining when the same sound pressure p is incident upon the fourth aperture and the pressure at all the other apertures is zero. The same analogy may be drawn with respect to apertures one and three. In terms of the electrical circuit shown in Figure 4, that is, (1) when V1=V2=V3==0; (2) when V1: V3=V4=0 and Vz==v, (where v=any arbitrary voltage), the potential difference across C2 will be twice the value obtaining when V1=V2=V3=O and V4=v; (3) but when V V2: V4=0 and V3=v, the potential difference across C1 will be twice the value obtaining when V2: V3=V4=0 and V1=v.

These conditions may be satisfied in different ways. For example, in the particular embodiment illustrated, the first condition is met by letting C1=C2, and R1=R4, R3=R2, L1=L4, La=L2. The second condition is then met by selecting the ratios Ri/Rs and L1/L3 so that i is twice as large when V1=Vz=V4=0 and Va=v, as it is when Vz=V3=V4=0 and V1=v. Under these conditions, it will be found that L1 should equal 2L3 and R1 should equal 2R3.

Althoughit is not necessary to keep the distance between the successive points at which the pressure is observed constant in order to obtain efficient operation of a given order, if these distances are not the same the calculation of the proper impedance relationships becomes more difiicult. The distances between the apertures should also be as large as possible in order to obtain high sensitivity. The limit on this distance is the effective phase differences between the points at which the pressure is observed when the distances between those points becomes an appreciable part of the wavelength. Therefore, insofar as both sensitivity and fidelity are concerned, it is generally most efficient to maintain the distances between the points at which the pressure is observed constant. For example, in the case of the third order microphone described herein, each of the tubes which occupies an extreme position is spaced apart from its next adjacent tube which occupies a mean position a distance equal to one-half the distance between the tubes occupying the mean positions.

From the foregoing description, it should be apparent that the present invention provides a noise-cancelling, pressure gradient responsive microphone which utilizes a minimum number of parts. In addition thereto, a micro-' phone may be constructed to provide second, third or higher order operation with only a single pressure-sensitive element.

Although only single embodiments of second and third order microphones in accordance with the present invention have been illustrated and described herein, it should be obvious to those persons skilled in the art that various changes and modifications arepossible within the spirit of the invention. For example, the pressure pick up points need not be disposed in a line but may be oriented in other fashion suitable to meet the requirements. However, random noise discrimination of a microphone having the pressure pick up points in a line will usually be greater than that of a microphone having its pick up points in any other orientation but which is otherwise identical. Therefore, it is desired that the particular forms of the present invention described herein shall be considered as illustrative and. not as limiting.

What is claimed is:

l. A third order, pressure gradient responsive microphone comprising a housing, a pressure-sensitive means mounted within said housing, means providing separate acoustical chambers on opposite sides of said. pressuresensitive means, and means. responsive to movement of said pressuresensitive means for converting the response of said pressure-sensitive means due to sound energy impinging thereon into corresponding electrical signals, said housing being provided with a plurality of separate means for picking up sound energy effectively at at least four points disposed in series spaced apart relation and for transmitting said sound energy to said acoustical chambers, said means for picking up sound energy at the first and last points in said series being so dimensioned as to provide an acoustical impedance which is substantially equal to twice the acoustical impedance of either of said means for picking up sound energy at the second and third points in said series, successive ones of said plurality of separate means being connected with alternate ones of said acoustical chambers.

2. A third order, pressure gradient responsive microphone comprising a housing, pressure-sensitive means mounted within said housing, means providing separate acoustical chambers on opposite sides of said pressuresensitive means, and means responsive to movement of said pressure-sensitive means for converting the response of said pressure-sensitive means due to sound energy impinging thereon into corresponding electrical signals, said housing being provided with a plurality of separate tubes, said tubes each having an end thereof disposed along a line and open to the ambient for picking up sound energy effectively at at least four points disposed in series spaced apart relation, the other ends of said tubes being connected with said acoustical chambers for transmitting said sound energy to said acoustical chambers, said tubes having ends at the first and last points in said series being so dimensioned as to provide an acoustical impedance substantially equal to twice the acoustical impedance of said tubes for picking up sound energy at the second and third points in said series, successive ones of the freely disposed ends of said tubes being connected with alternate ones of said acoustical chambers.

3. A third order, pressure gradient responsive microphone comprising a housing, pressure-sensitive means mounted within said housing, means providing separate acoustical chambers on opposite sides of said pressure sensitive means, and means responsive to movement of said pressure-sensitive means for converting the response of said pressure-sensitive means due to sound energy impinging thereon into corresponding electrical signals, said housing being provided with a plurality of tubes connected at one end thereof with said acoustical chambers and having the other end thereof freely disposed and open to the ambient for picking up sound energy effectively at at least four points disposed in series spaced apart relation and for transmitting said sound energy to said acoustical chambers, said tubes for picking up sound energy at the first and last points in said series being so dimensioned as to provide an acoustical impedance which is substantially equal to twice the acoustical impedance of either of said means for picking up sound energy at the second and third points in said series, successive ones of said plurality of tubes being connected with alternate ones of said acoustical chambers.

References Cited in the file of this patent UNITED STATES PATENTS 2,301,638 Olson Nov. 10, 1942 2,529,467 Wiggins Nov. 7, 1950 2,552,878 Wiggins May 15', 1951 FOREIGN PATENTS.

594,646 Great Britain Nov; 17,1947 

